Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95A (TIA/EIA IS95A) herein after referred to as IS-95A. In accordance with IS-95A, the coded communication signals used in the DS-CDMA system comprise signals that are transmitted in a common 1.25 MHz bandwidth to base sites of the system from communication units, such as mobile or portable radiotelephones, that are communicating in the coverage areas of the base sites. Each DS-CDMA signal includes, inter alia, a pseudorandom noise (PN) binary code associated with a particular base site and a PN sequence associated with a particular mobile station.
During a typical communication, a mobile station travels within a coverage area of a base site that is supporting the mobile station communication. Such movement typically results in fading of the mobile communication signal transmitted to the base site due to multipath propagation of the transmitted signal. As is known, multipath propagation results from the reflections of the transmitted signal off of nearby scatterers, such as buildings. These reflections produce replicas of the originally transmitted signal which arrive at the base site at various times depending on the effective propagation distances traveled by the replicas. The originally transmitted signal and its reflected replicas are typically referred to as multipath signals of the originally transmitted signal.
To demodulate the transmissions from a particular mobile station, a base site receiver must first differentiate the multipath signals associated with a particular mobile station from other multipath signals as well as simple noise associated with other mobile stations. This identification of a particular mobile station's multipath signals, including their locations with reference to an offset in time (referred to as a pn-offset), may be referred to as an antenna, or signal search, which takes advantage of well known principles of time-diversity demodulation. In a CDMA system, the pn-offset is measured in increments of time referred to as pseudorandom noise (PN) chips, wherein each PN chip is approximately equivalent to 80 milliseconds.
In a sectorized antenna system, there are generally two antennas for each of the sectors. For a typical time-diversity demodulation process, the initial searching process begins in a searcher. The searcher identifies the strongest received multipath signals via correlating PN sequences with the incoming multipath signal over a range of pn-offsets. The searcher then calculates the received multipath signal energies over the range of pn-offsets, incremented in 1/2 PN chip steps, for a set of antennas but does not demodulate or decode the information at these offsets.
The search is set such that the antennas within a sector are searched serially. After all the antennae have been searched, the searcher outputs two arrays of eight data points, referred to as winning Walsh energies at their associated pn-offsets. The resulting pn-offsets are then compared, using a finger assignment algorithm, to the previously selected pn-offsets for the four RAKE receiver demodulator fingers. The previously selected pn-offsets may or may not be replaced by one or more of the resulting sixteen pn-offsets, depending on the finger assignment algorithm. Accordingly, the four RAKE receiver demodulator fingers track and demodulate the multipath signals at their associated pn-offsets.
A delay lock loop (DLL) algorithm in each RAKE receiver demodulator finger serves to further correlate the multipath signal energies using pn-offset increments of 1/8 PN chips thus yielding 1/8 PN chip resolution.
In urban areas, two or more multipath signals arriving at the same antenna may have pn-offsets which are within 1 PN chip apart. When this occurs, the advantage of time-diversity demodulation is diminished because of the increased correlation between the two multipath signals and the 1/2 PN chip resolution limitation of the searcher. For example, field testing has indicated that at 900 megahertz (MHz), base site RAKE receiver performance measured as full-rate frame erasure rate (FER), was substantially degraded when two multipath signals arrived at one antenna with pn-offsets which were equal to, or less than one Walsh chip apart.
Therefore, a need exists for a method and apparatus for tracking a communication signal in a wireless communication system which improves the prior art tracking algorithm.